1. Field of the Invention
The present invention relates to a communication apparatus and a communication method using CDMA (Code Division Multiple Access).
2. Description of the Related Art
FIG. 1 illustrates a conventional mobile communication apparatus for executing base station transmission diversity using CDMA. In FIG. 1, a base station has four base station antennas and is concurrently communicating with three mobile stations in a cell. In other words, base station 1 has four antennas (1) through (4), 2 through 5, and three mobile stations 6 through 8 in communicating with base station 1 exist in a cell. Base station 1 executes transmission diversity by switching four transmission antennas (1) through (4).
In a radio communication, propagation conditions of radio signals vary largely depending on transition of a mobile station or a surroundings material. If there are enough spaces between four antennas (1) through (4), each propagation condition of radio signals of mobile station (1) with each of four antennas (1) through (4) varies independently. In mobile station (2) and (3), the conditions are also in the same way.
In a TDD (Time Division Duplex) system for communicating alternatively in forward link and reverse link at certain intervals in the same radio frequency band, high similarity is maintained in radio propagation conditions, which allows to easily assume the propagation condition of reverse link from the propagation condition of reverse link.
At a certain time, at a base station, after comparing the radio propagation conditions of mobile station (1) with base station antennas (1) through (4), in the case where the propagation condition of radio signals with antenna (3) is assumed to be the best, information signals for mobile station (1) are transmitted from antenna (3). Similarly, in the case where the propagation condition of radio signals with antenna (3) at mobile station (2) is assumed to be the best, information signals for mobile station (2) are transmitted from antenna (3), and in the case where the propagation condition of radio signals with antenna (2) at mobile station (3) is assumed to be the best, information signals for mobile station (3) are transmitted from antenna (2).
In this way, since information signals for each mobile station are transmitted from an antenna with the best condition in radio propagation, it is possible to improve radio communication quality and increase the number of mobile stations capable of concurrently communicating with a base station at the same quality level.
However, in a conventional mobile communication apparatus for executing base station transmission diversity described above, since one base station transmission antenna is selected, in the case where a wrong transmission antenna is selected or the propagation condition of radio signals varies faster than a selection period of transmission antenna, it may occur that information signals are not transmitted from an antenna under good propagation condition, which results in deterioration of communication quality. And, since same spread signals are transmitted from a plurality of antennas at the same time, interference in the signals causes the different propagation condition between transmission and reception, which brings the problem that an antenna in good propagation condition is not selected for transmission.